Vital to any telecommunication network is the manner in which its busiest points manage great quantities of bandwidth. These points usually occur at large central offices (COs). At COs, enormous amounts of traffic must be switched between enormous numbers of possible input or output links of varying data capacities.
Space-switches are vital to achieving this function, since they interconnect the large numbers of input and output links that converge at a CO. Presently, these space-switches are typically implemented using digital cross-connect (DCS) technology. DCS switches switch signals that are in electrical form and that conform to one of a set of digital signal formatting protocols. The highest capacity signal that can be switched by a DCS switch complies with the DS3 protocol, which supports a data rate of 44.736 MBPS. DCS switches capable of supporting this maximum rate are referred to as DCS 3/3 switches, and are widely used in large COs.
With the emergence of optical fibres as the principal transport medium for carrying telecommunication signals from point to point, the exclusive use of DCS space-switches is rapidly becoming an undesirable means by which to space-switch large quantities of traffic within very busy COs. This is firstly because the signals sent through fibre optic media are in optical form rather than in electrical form, which means that the use of DCS switches to guide a signal from an optical input link to an optical output link mandates intermediate equipment between the DCS switch and the transport switch terminating such optical links, that convert the signals from an optical form to an electrical form for switching purposes, and then back to an optical form for transport purposes. Such equipment requires additional cabling, power and space, and adds complexity to the CO and its components.
A second reason why the use of DCS switches is becoming unsuitable for space-switching in busy COs, is because the ports of such switches have not been cheaply adapted to switch individual signals at rates higher than the rate supported by the DS3 protocol (44.736 MBPS). This is a problem because all optical signals comply with protocols, such as the Synchronous Optical Network (SONET) protocols, which call for rates many times greater than 44.736 MBPS. The limit on the per-port throughput of DCS switches means that the only way to use such a switch to space-switch a signal at a rate exceeding 44.736 MBPS, is by demultiplexing the high rate signal down to several DS3 signals for switching purposes, switching the DS3 signals on the DCS 3/3 switch, and then remultiplexing the DS3 signals back into a single higher-rate signal for transport purposes. Like the first problem, this mandated practice also requires additional resources for, and adds increased complexity to, the CO and its components.
Even more significantly, this limitation in the per-port rates supported by DCS switches places limits on the total bandwidth that can be managed by a CO, which relies only on DCS switches to interconnect various pieces of network elements. More specifically, this limitation means that a large number of DCS 3/3 ports are required to space-switch a signal operating at a rate higher than the DS3 rate. For example, 48 input and output DCS 3/3 ports are required to connect a single OC-48 link to other parts of the CO. Support for a plurality of such high-rate links can thus quickly lead to the utilisation of all ports on an existing DCS 3/3 space switch, which only comes in sizes larger than 1000 ports by 1000 ports at a great cost, and which does not come in sizes larger than 2000 ports by 2000 ports. Once all the ports on a space-switch are utilised, no further interconnections between links that converge at a CO are possible. This in turn imposes a limit on the amount of bandwidth that can flow through the CO.
Cascading DCS switches to form single higher-capacity space switches does not significantly alleviate this last problem, as no further increases in switching capacity are realised once more than a few DCS switches have been interconnected. More specifically, as the number of DCS switches comprising a single larger block of DCS switches increases, each additional DCS switch that is added to the block must set aside an increasing proportion of its ports just to communicate with the other DCS switches in the block, rather than to connect new network links into the CO. At some point, the addition of an additional DCS switch would fail to add any cross-connect switching capacity to the CO, as the additional DCS switch would have to allocate all of its ports just for communication with the other DCS switches that are already in the block. Therefore, even if deployed as interconnected groups, the use of DCS switches impose a limit on the total bandwidth that can be handled by a CO.
A solution to the problems created by the use of DCS switches is thus required. The use of electrically controlled optical signal cross-connect (OXC) switches, in which an output fibre carries the same light that entered the switch on an input fibre, could eventually be a complete solution to the problem of managing large amounts of bandwidth at large central offices. By switching signals in their optical forms, these switches avoid the signal conversion problems associated with DCS technology. Furthermore, because such a switch is capable of interconnecting links operating at any of the high-capacity SONET rates, it can also increase the bandwidth capacity of the CO.
At present however, problems with cross-talk, the lack of a memory function when processing optical signals, and a lack of reliability, are restricting the size of these OXC switches to 16 ports by 16 ports, and preventing them from being interconnected to form a larger switch. Furthermore, OXC switches are still very costly. Additionally, there are no known methods for performing several important bandwidth management services on optical signals, such as traffic grooming (e.g. the ability of intermediate nodes to split traffic coming from a common source to two separate destinations), time-slot interchanging (TSI), and performance monitoring and testing of paths, lines and sections, which are routinely performed on electrical signals. Therefore, an OXC switch by itself is not a desirable solution to the aforementioned bandwidth management problems at present.
The foregoing indicates that a solution in the manner in which bandwidth is managed at COs, is immediately required until the capabilities of OXC-switches can be expanded. Besides alleviating or mitigating some or all of the above-identified problems, this interim solution must also minimise the investment that must be made in soon-to-be obsolete switching technology.
One proposed interim solution to these problems, involves providing optical interfaces into a DCS switch. This would internalise much of the signal processing required to electrically switch signals that arrive at and leave from the CO in optical form, within the DCS switch. It also allows for a reduction in the number of cables required to connect network elements to the DCS switch. Moreover, since the DCS switch would still switch electrical signals, the CO would still be able to support a variety of bandwidth management services.
At present however, the highest available interfaces support only the OC-12 rate. DCS switches with OC-12 interfaces are also very costly.
Many high capacity transport nodes, such as SONET nodes, provide several standardised services that are also implemented on DCS switches. These include electrical space switching at the DS3 rate, time-slot assignment (hair-pinning), time-slot interchange (TSI), traffic grooming, and performance monitoring and testing of paths, lines and sections.
In well-provisioned networks, all the COs are connected in a full or partial mesh using redundant links for protection switching purposes. Such a mesh is said to implement an alternate path of restoration service, which provides contingency routes between COs in the event of a failure in the network.